The present invention relates to a nonaqueous battery, particularly a lithium ion battery, and a nonaqueous battery containing the electrolyte.
The development of electronic technology in recent years is remarkable, and various apparatus and devices have been reduced in size and weight. Accompanying the reduction in size and weight of such electronic apparatus and devices, there has been a remarkably increasing demand for reduction in size and weight of a battery as a power supply for such electronic apparatus and devices. As a battery capable of providing a large energy at small volume and weight, a nonaqueous secondary battery using lithium has been used as a power source for principally small-sized electronic appliances, such as portable telephone sets, personal computers and video cam coders, used at home. For the purpose of providing the lithium nonaqueous secondary battery with increased shape latitude, e.g., formation into a very small thickness on the order of 0.5 mm, extensive development work has been made on polymer electrolyte batteries.
A polymer electrolyte containing no electrolytic solution hardly satisfies properties required for application to batteries because of, e.g., low ionic conductivity and small battery discharge capacity. In contrast thereto, a polymer gel electrolyte containing electrolytic solution has called an attention because of a high ionic conductivity. As such a polymer electrolyte, U.S. Pat. No. 5,296,318 has reported a polymer electrolyte using a copolymer of vinylidene fluoride with 8 to 25 wt. % of hexafluoropropylene. Further, as a technique for impregnating the copolymer with an increased amount of electrolytic solution, U.S. Pat. No. 5,456,000 has disclosed a technique of mixing the copolymer with a plasticizer, then extracting the plasticizer and then effecting the impregnation with a nonaqueous electrolytic solution. According to this technique, it is possible to effect the impregnation with a large amount of electrolytic solution, but such impregnation with a large amount of electrolytic solution is accompanied with a problem of losing a shape latitude, such as the formation into a very small thickness. Further, as the technique essentially involves a step of extracting the plasticizer, the productivity becomes inferior. Further, complete extraction of a plasticizer is difficult, and a portion of the plasticizer remaining in the polymer electrolyte is liable to exert an adverse effect to the battery prepared by using the electrolyte.
In order to obtain a polymer electrolyte battery having a high shape latitude, it is essential to provide a polymer gel electrolyte capable of containing a large amount of electrolytic solution so as to enhance the ionic conductivity and yet exhibiting a large strength. However, the strength of a gel is lowered at a larger content of electrolytic solution, so that it has been impossible to satisfy a gel strength and a content of electrolytic solution in combination, and no polymer gel electrolyte suitable for providing a polymer electrolyte battery having a high shape latitude has been known.
In order to increase the gel strength, it is considered important to provide an enhanced modulus of elasticity to the gel. Factors controlling the elasticity modulus of a gel have been generally obscure except that a higher polymer concentration provides a higher elasticity modulus (but this results in a lower content of electrolytic solution in the polymer electrolyte and is thus not practical), and it has been reported that an increase in polymer molecular weight does not result in a change in elasticity modulus with respect to xcexa carrageenans gel by Rochas, C. et al, Carbohydrate Polymers, 12, 255-266 (1990). In this way, as general guiding principles for enhancing the gel strength, none have been known except for relying on a higher polymer concentration. Accordingly, a practical polymer electrolyte capable of being impregnated with a large amount of nonaqueous electrolytic solution and yet having an excellent strength, has not been known.
Further, in the case of being impregnated with a large amount of nonaqueous electrolytic solution, it is necessary to stably retain the solution and prevent the solution from leaking out of the polymer electrolyte. If the nonaqueous electrolytic solution cannot be stably retained and a large amount of leakage thereof is caused, it becomes impossible to obviate damages and deterioration of electrical properties of apparatus and devices surrounding the battery.
The present invention aims at providing a polymer electrolyte capable of being impregnated with a large amount of nonaqueous electrolytic solution and stably retaining the electrolytic solution and yet exhibiting excellent strength, and further a nonaqueous battery having a large shape latitude by using the polymer electrolyte.
According to the inventors"" study for accomplishing the above objects, it has been found very preferable to use a polymer electrolyte, comprising: a vinylidene fluoride copolymer and a nonaqueous electrolytic solution, wherein the vinylidene fluoride copolymer comprises 80 to 97 wt. % of vinylidene fluoride monomer units and 3 to 20 wt. % of units of at least one monomer copolymerizable with vinylidene fluoride monomer and has an inherent viscosity of 1.5 to 10 dl/g. Herein, xe2x80x9cinherent viscosityxe2x80x9d is used as a measure of polymer molecular weight and refers to a logarithmic viscosity number as measured at 30xc2x0 C. of a solution formed by dissolving 4 g of a polymer resin in 1 liter of N,N-dimethylformamide.